Radar device measures at least one of a distance and a direction of a target from a measuring place by radiating radio waves to the space from the measuring place and receiving a pulse signal of reflection waves reflected by the target. In particular, in recent years, radar device which can detect targets including automobiles and pedestrians by a high-resolution measurement using short-wavelength radio waves including microwaves and millimeter waves have been being developed.
There may occur a case that a radar device receives a signal of a mixture of reflection waves coming from a nearby target and reflection waves coming from a distant target. In particular, where range sidelobes occur due to a signal of reflection waves coming from a nearby target, the range sidelobes and a main lobe of reflection waves coming from a distant target exist in mixture, as a result of which the accuracy of detection of the distant target is lowered in the radar device.
Therefore, radar device using a pulse signal which need to perform high-resolution measurements on plural targets are required to transmit pulse waves or pulse modulation waves having an autocorrelation characteristic with low range sidelobe levels (hereinafter referred to as a low range sidelobe characteristic).
When an automobile and a pedestrian are located at the same distance from a measuring place, a radar device receives a signal that is a mixture of signals of reflection waves coming from the automobile and the pedestrian which have different radar cross sections (RCSs). In general, the radar cross section a pedestrian is smaller than that of an automobile.
Radar devices are required to properly receive signals of reflection waves coming from an automobile and a pedestrian even if they are located at the same distance from a measuring place. The reception signal level of a reflection wave signal varies depending on the distance and the type of a target. Radar devices are required to have such a reception dynamic range as to be able to receive reflection wave signals having various reception signal levels.
In conventional radar device using pulse compression, a technique is known which increases the SNR (signal-to-noise ratio) of reception of reflection waves coming from a target by adding together correlation values calculated by pulse compression processing in the case where a pulse compression code is transmitted repeatedly at a transmission cycle Tr. The addition is classified into coherent integration and non-coherent integration (also called incoherent integration).
For example, in a period (Nc×Tr) with high time correlation, I components and Q components of correlation values calculated by pulse compression processing can be separately subjected to coherent integration. The parameter Tr is the pulse transmission cycle (s). As is understood from Formula (1), the coherent integration makes it possible to improve the reception quality by a coherent integration gain Gc (dB) from a reception SNR (dB). The coherent integration gain Gc is calculated according to Equation (2):[Formula 1]SNR[dB]+Gc[dB]  (1)[Formula 2]Gc=10 log10(Nc)[dB]  (2)
The parameter Nc, which is the coherent integration number, is set depending on an assumed maximum movement speed of a target. Therefore, as the assumed maximum movement speed of a target increases, the variation of a Doppler frequency included in a signal of reflection waves coming from a target is increased and the period with high time correlation becomes shorter. As the coherent integration number Nc decreases, the coherent integration gain Gc is decreased as seen from Equation (2), that is, the SNR enhancement effect of the coherent integration is decreased as seen from Formula (1).
On the other hand, also in the case of the non-coherent integration, as is understood from Formula (3), the SNR can be increased by adding together amplitude or reception power components of correlation values calculated by pulse compression processing. The parameter Gd is the non-coherent integration gain and is calculated according to Equation (4):[Formula 3]SNR[dB]+Gd[dB]  (3)[Formula 4]Gd=10 log10(√{square root over (Nd)})[dB]  (4)
The parameter Nd is the non-coherent integration number. Where the coherent integration number Nc and the non-coherent integration number Nd are the same, as is understood from Equations (2) and (4), the coherent integration contributes more to the gain increase than the non-coherent integration. However, to obtain a gain of ideal coherent integration, it is necessary that the phase component of a reception signal be kept constant in a prescribed range. The range where the coherent integration is possible is thus restricted.
When Fourier transform is performed using Nf correlation values calculated by pulse compression processing at a particular discrete time point for an interval (Nf×Tr) of Nf transmissions of a pulse compression code, a Doppler spectrum included in reflection waves coming from a target can be observed from a Fourier-transformed frequency domain signal. In conventional radar device using pulse compression, a signal component in which the gain is increased by the coherent integration can be detected from a peak frequency component (hereinafter referred to as “peak Doppler spectrum”) in a Doppler spectrum. An FFT (fast Fourier transform) or DFT (discrete Fourier transform) algorithm is used for the Fourier transform. Although in the following description Fourier transform will be abbreviated as FFT, the same discussions will hold even if FFT is replaced by DFT.
FFT processing which is performed using Nf correlation values calculated by pulse compression processing at a particular discrete time point for an interval (Nf×Tr) of Nf transmissions of a pulse compression code will be referred to as “coherent integration by FFT” or “FFT coherent integration.”
In conventional radar device using pulse compression, where reflection waves from a target include a phase variation due to a Doppler frequency shift that is caused by a movement of the target, coherent integration that conforms to the phase variation due to the Doppler frequency shift is enabled if a peak of a Doppler spectrum is detected by coherent integration by FFT.
In conventional radar device using pulse compression, if the spread of a Doppler spectrum (Doppler spread) is sufficiently small, the coherent integration has a gain increase effect (see Formula (1)) irrespective of the coherent integration interval which corresponds to the FFT size. In particular, in conventional radar device using pulse compression, in the case where the Doppler spectrum can be approximated by a line spectrum, the gain Gf (dB) of the coherent integration effect is obtained as given by Equation (5), where Nf is the FFT coherent integration number:[Formula 5]Gf=10 log10(Nf)[dB]  (5)
To obtain the ideal coherent integration gain given by Equation (5) in conventional radar device using pulse compression, it is necessary that the Doppler spread δd which depends on a phase variation included in reflection waves coming from a target be sufficiently small. That is, in conventional radar device using pulse compression, the coherent integration gain decreases as the Doppler spread δd which depends on a phase variation included in reflection waves coming from a target increases.
FIG. 17(a) shows relationships between Doppler spectrum characteristics and Doppler spreads after FFT. The horizontal axis represents the frequency and the vertical axis represents the Doppler spectrum. FIG. 17(b) shows a relationship between the coherent integration gain and the number of times of coherent integration (integration interval) which corresponds to the FFT size with the Doppler spread as a parameter. The horizontal axis represents the number of times of coherent integration (logarithmic scale) and the vertical axis represents the coherent integration gain.
As seen from FIG. 17(b), when the Doppler spread δd is large, the coherent integration gain starts to be saturated when the number of times of coherent integration is equal to A. When the Doppler spread δd is small, the coherent integration gain starts to be saturated when the number of times of coherent integration is equal to B. Therefore, the number of times of FFT coherent integration corresponding to the saturation start point of the coherent integration gain decreases as the Doppler spread δd becomes large.
In conventional radar device using pulse compression, when the Doppler spread δd is large and the coherent integration gain starts to be saturated early even if the coherent integration number is increased (described above), the SNR can be increased more by using both of coherent integration and non-coherent integration and increasing the number of times of non-coherent integration.
The Doppler spread which is included in reflection waves coming from a target is larger when more reflective objects exist around a place where the target is located or the target itself has more scattering points. And the Doppler spread which is included in reflection waves coming from a target tends to increase as the directivity of a transmission antenna or a reception antenna becomes wider.
In conventional radar device using pulse compression, the following problem arises if the number Nf of times of FFT coherent integration is set as a fixed vale in the case where targets (positioning targets) include a target which produces a large Doppler spread and a target which produces a small Doppler spread and reflection waves coming from the targets thus have a wide range of Doppler spreads.
Where positioning of a target having a large Doppler spread is used as a reference, if the coherent integration number is set smaller than the non-coherent integration number, one cannot enjoy a coherent integration gain which should be obtained in positioning of a target having a small Doppler spread.
Conversely, where positioning of a target having a small Doppler spread is used as a reference, if the coherent integration number is set larger than the non-coherent integration number, the SNR cannot be increased sufficiently in positioning of a target having a large Doppler spread due to saturation of the coherent integration gain (see FIG. 17(b)).
For example, a radar device disclosed in Patent document 1 is known as a countermeasure against the above problem. This radar device is configured so as to be equipped with plural range gates the width of each of which is determined by a pulse width, and to include plural coherent integrators, plural wave detectors, plural non-coherent integrators, and plural threshold detectors in such a manner that they correspond to the respective range gates.
It is also disclosed that, in this radar device, a target is detected for each range by performing, with the plural coherent integrators and the plural non-coherent integrators, plural kinds of integration processing which are different in the ratio between the coherent integration number and the non-coherent integration number and comparing plural signals obtained by these kinds of integration processing with prescribed threshold values with the plural threshold detectors.